1. Field of the Invention
The present invention generally relates to a small boat comprising a plurality of propulsion units. More particularly, the present invention relates to a control device to simplify operation of the small boat while conducting turning operations.
2. Description of the Related Art
An outboard motor mounted to a transom of a small boat functions as a steering device. A drive unit of the outboard motor pivots about an axis of a swivel shaft by a turning angle dictated by a steering angle of a steering wheel. When thrust is applied to the hull of the boat, e.g., the outboard motor pushes against the transom, the boat turns in accordance with the turning angle.
An electrically operable steering device that interconnects the outboard motor to the steering wheel is disclosed in Japanese Patent Document JP-B-2959044. By the use of the electrically operable steering device, a motor is driven to cause the outboard motor to turn as directed by the steering angle of the steering wheel. Thereby, steering can be easily accomplished. While the boat is turning, the outboard motor applies a side thrust to the transom.
The output of the outboard motor is adjusted by manipulating an accelerator lever provided in a cockpit together with the steering wheel. The accelerator lever has a neutral range that covers a certain angle of a central portion of the control device. When the lever is pivoted forward from the neutral range, the lever moves to a forward shift position and a throttle valve opens to correspond to an angle of the lever (e.g., more of an angle at the lever results in a more open throttle valve), thereby increasing the output of the outboard motor to move forward. Conversely, when the lever is pivoted rearward, the lever moves to a reverse shift position and the output of the outboard motor is varied.
If a boat has two outboard motors mounted to the transom side by side, each of the outboard motors has an accelerator lever of its own so that the output of the respective outboard motors can be individually adjusted.
FIGS. 8(A) and 8(B) illustrate turning operations of a boat having multiple outboard motors (two outboard motors in this illustration). FIG. 8(A) shows a large turning angle turning operation, while FIG. 8(B) shows a small turning angle turning operation.
In FIG. 8(A), pivotal moments M1, M2 are made around the center of turn by thrust (F) generated in accordance with the output of the outboard motors 3a, 3b, which are mounted to a transom board 2 of a hull 1. With small turning angles, such as that shown in FIG. 8(B), the pivotal moments M3, M4 made around the center of turn by the thrust (F) of the respective outboard motors 3a, 3b affect the hull 1 in opposite directions relative to each other. Therefore, in such a state, the turning operation can be more efficiently made if the output of the outboard motor 3a is reversed (i.e., the reverse shift position). If, however, the output is reversed, the total forward thrust is reduced. The smaller thrust does not significantly effect low speed operation. At higher speed operation, energy loss may be noticed by an operator of the small boat. Also, it is almost impossible to shift one of the outboard motors to the reverse mode in a high speed operational range close to full throttle operation.
Accordingly, when two outboard motors are used, the accelerator levers of the respective outboard motors typically are operated individually depending upon the running condition, such as, for example, a steering angle, a speed or an acceleration corresponding to the steering angle and/or a shift position, while the steering angle is given by the steering wheel; however, such a configuration can be improved.
Japanese Patent Document JP-A-Hei 1-285486 discloses a boat control device by which thrust directions and magnitudes of two propulsion units can be optimized. The control device of Japanese Patent Document JP-A-Hei 1-285486 has an omni directional commanding device such as, for example, a joystick instead of a steering wheel. The steering angles of the respective propulsion units are varied in accordance with the directions given by the joystick and the thrusts thereof are also changed. Thus, the boat is turned in accordance with the commands given through the joystick.
The control device of Japanese Patent Document JP-A-Hei 1-285486 is complicated due to the addition of the joystick to the steering wheel. The control device changes the turning directions of the respective propulsion units. That is, the control device changes the directions of the individual thrusts of the propulsion units in a horizontal plane. Because of this feature, the individual thrusts can cancel each other under certain conditions depending upon the turning radius of the boat or a speed of the boat. Cancelling of the thrusts can cause perceptible energy loss.